The immune system is the body's natural defense system that helps fight infections. The immune system is made up of antibodies, white blood cells, and other chemicals and proteins that attack and destroy substances such as bacteria and viruses that they recognize as foreign and different from the body's normal healthy tissues.

The immune system is also responsible for allergic reactions and allergies, which may occur when the immune system incorrectly identifies a substance (allergen), such as pollen, mold, chemicals, plants, and medicines, as harmful.

Sometimes the immune system also mistakenly attacks the body's own cells, which is known as an autoimmune disease.